It is customary to conduct reflection seismographic measurements by producing acoustic waves which pass through the earth and are reflected, received and analyzed. Normally, such waves are produced by blasting or with one or more vibrators in or on the earth's surface, the conventional sources producing vibrations which are essentially pressure waves (P-waves) and such waves are received with conventional geophones which can be, for example, vertically disposed plunger-coil assemblies or similar devices responsive to waves reflected from interfaces in the sub-soil. It is possible to supplement these measurements by the production and reception of shear waves in which case it is possible to calculate the Poisson constant for individual characteristic reflections from the quotient of the speeds of the P- and S-waves which, in turn, can be analyzed to produce further information about the lithological characteristics of the reflection interfaces. Exhaustive reports concerning the production and reception of shear waves are printed in "Geophysics 1968", pages 229-254, including additional references to literature. The efforts along these lines depend primarily on the use of special vibrators such as those discussed in U.S. Pat. Nos. 3,159,232; 3,159,233; and 3,205,971. Such techniques employ instruments which are built for the special purpose of producing S-waves. They permit the production, of shear waves of the type SH, that is, with the polarization in the plane of the reflection horizon. With the use of correspondingly polarized geophones, good shear wave seismograms have been recorded in these reported experiments.
The vibrators used in the known experiments as shown, for example, in the patents already mentioned, fulfill the physical requirements for this special type of reflection seismic recording in an almost ideal manner. However, in the practical application of these devices, the vibrators have significant disadvantages. Because of the horizontal movement of the vibrator mass, the elements used for the guidance of the mass are very much subject to wear. For the purpose of transferring the shear forces to the sub-soil, the known vibrators are equipped with a steel plate which has pyramid-shaped warts or projections on that surface by which the plate is coupled to the soil. When using these vibrators, especially on highways and roads, expensive damage to the coupling surfaces is caused. A further and very significant disadvantage of these vibrators is that, with the vibrator performance used at the present time and considering the mass necessary therefor, it is not easily possible to insert such a vibrator into equipment which is customarily available for the production of P-waves.
Generally, the experiments, continued until the most recent times in the United States with shear waves for seismic trial operations, have shown that good results can be achieved under the most variable geological and terrain conditions. In almost all areas with the exception of loose sand and water-saturated sediments, shear wave vibrators per se may be used as a source for the production of shear waves of the type SH. At the same time, in many areas, shear wave signals are observed in sufficiently great multiplicity. However, hitherto, the multiplicity has always been at least twice as great as in the case of recording of compression waves. In connection with the recordings heretofore, evaluating and interpreting processes have been developed which, as pointed out above, contribute to the discovery of lithological anomalies.
It will be apparent that special so-called horizontal geophones are needed for the recording of SH waves when they are the primary waves used for sub-soil investigation. Providing testing crews with additional horizontal geophones is much less of a problem that the need for providing special devices for the production of S-waves.